a) Field of the Invention
The present invention relates to speed ratio (or, gear ratio) control apparatus and method for a hybrid transmission, in which an engine, an output member, a first motor/generator, and a second motor/generator are coupled separately (or, independently) to different rotational elements in a two-degree-of-freedom differential mechanism having at least four rotational elements.
b) Description of the Related Art
A Japanese Patent Application First Publication No. 2003-34154 published on Feb. 4, 2003 exemplifies a previously proposed hybrid transmission speed ratio control apparatus in which the speed ratio is provided by means of two motor/generators. During a vehicle driving with an accelerator manipulated variable kept constant, the speed ratio is controlled by maintaining an engine speed at a target value. Moreover, when the accelerator is released by a driver, the vehicle is decelerated with the speed ratio controlled in accordance with a traveling state (i.e., a vehicle velocity or a target driving force).